Presentation on theme: "Convex and Concave Lenses"— Presentation transcript:
1Convex and Concave Lenses Types of LensesA lens is a piece of transparent material, such as glass or plastic, that is used to focus light and form an image.Each of a lens’s two faces might be either curved or flat.
2Convex and Concave Lenses Types of LensesThe lens shown in the figure is called a convex lens because it is thicker at the center than at the edges.A convex lens often is called a converging lens because when surrounded by material with a lower index of refraction, it refracts parallel light rays so that the rays meet at a point.
3Convex and Concave Lenses Types of LensesThe lens shown in the figure is called a concave lens because it is thinner in the middle than at the edges.A concave lens often is called a diverging lens because when surrounded by material with a lower index of refraction, rays passing through it spread out.
4Ray diagrams for lenses Ray diagrams are drawings of the different situations for lenses. For the ray diagrams, assume that the lenses are thin.
5Converging Lenses Principal focus or Focal point Principal axis Optical centreFocal planeFocal length
7Convex and Concave Lenses Convex Lenses and Real ImagesPaper can be ignited by producing a real image of the Sun on the paper.The rays of the Sun are almost exactly parallel when they reach Earth.
8Convex Lenses and Real Images After being refracted by the lens, the rays converge at the focal point, F, of the lens.The figure shows two focal points, one on each side of the lens.You could turn the lens around, and it will work the same.
9Ray diagrams for a double convex lens Object is at infinityF
15Images Formed by Lens Object distance Type of image Uses u = Inverted, smaller, realTelescopeu > 2fCamera, eyeu = 2fInverted, same size, realPhotocopierf < u < 2fInverted, magnified, realProjectoru = fupright, magnified, realSpotlightu < fupright, magnified, virtualMagnifying glass
16Lens EquationsThe thin lens equation relates the focal length of a spherical thin lens to the object position and the image position.The inverse of the focal length of a spherical lens is equal to the sum of the inverses of the image position and the object position.
17Thin Lens Equation The thin lens equation is stated as follows: where do is the distance (measured along the axis) from the object to the center of the lensdi is the distance (measured along the axis) from the image to the center of the lensf is the focal length of the lensThe expression 1/f is called the power of a lens. It is measured in Diopters, where 1 D = 1 m-1.
18Lens EquationsThe magnification equation for spherical mirrors also can be used for spherical thin lenses.It is used to determine the height and orientation of the image formed by a spherical thin lens.The magnification of an object by a spherical lens, defined as the image height divided by the object height, is equal to the negative of the image position divided by the object position.
19do is always positive with a single lens di is positive for real images, negative for virtual images f is positive for converging lenses, negative for diverging lenses
20When using this equation, signs are very important: Remember that do, When using this equation, signs are very important: Remember that do,di, and f must be measured in the same unit - usuallymeters is preferred.dopositivewhen the object is placed "in front of the lens"diwhen real images are formed (inverted, "behind the lens")negativewhen virtual images are formed (upright, "in front of the lens")fwhen the lens is convergingwhen the lens is diverging
21Concave Lenses A concave lens causes all rays to diverge. The figure shows how such a lens forms a virtual image.
22Concave LensesThe image is located at the point from where the two rays apparently diverge.The image also is upright and smaller compared to the object.object
23Convex and Concave Lenses Ray 1 approaches the lens parallel to the principal axis, and leaves the lens along a line that extends back through the focal point.Ray 2 approaches the lens as if it is going to pass through the focal point on the opposite side, and leaves the lens parallel to the principal axis.
24Concave LensesThe sight lines of rays 1 and 2 intersect on the same side of the lens as the object.Because the rays diverge, they produce a virtual image.